The present invention relates generally to welding, and, more specifically, to butt welding.
A ceramic metal halide (CMH) lamp includes a cylindrical arc tube having substantially closed ends from which extend smaller tubular sockets. An electrode assembly is mounted in each of the sockets and includes an electrode tip suspended inside the arc tube. The tube is filled with a suitable gas and the sockets are sealed so that when electrical current is conducted through the electrode assemblies a light emitting arc is produced inside the tube. Since the operating temperature of the lamp is substantial, the electrode assemblies must be formed of suitable materials for withstanding the elevated temperature.
For example, the electrode tip may include a shank and coil of tungsten. The tungsten tip is joined coaxially to refractory or ceramic-metal composite lead wires which extend through the tube sockets. In one embodiment, the tungsten tip is joined to a molybdenum wire, or a wire formed of cermet which is an alloy of molybdenum and alumina. This wire in turn is joined to a niobium wire for forming a three-part electrode assembly that extends through each of the two tube sockets.
The components of the electrode assembly should be joined together with suitable strength for withstanding the high temperature operation of the lamp, which may be effected by welding together the three parts at two corresponding welding joints. Experience has shown that higher strength welds of these parts may be obtained by imposing an axial compression load between the parts during the welding process. In this way, the parts are forced together during the welding process to ensure intimate contact therebetween throughout the welding process for achieving a complete welding bond without the use of a filler material.
However, since one or both of the two parts being welded together at each joint will locally melt, the applied compression force can cause undesirable plastic mushrooming at the weld joint with radially outward expansion thereat. The resulting weld joint, or nugget, is correspondingly larger in diameter than that of the adjacent parts.
Such weld nuggets are undesirable in the manufacture of CMH lamps since the lamp sockets are relatively small in diameter for receiving therethrough the electrode assemblies in a close-tolerance fit. For example, the inner diameter of the tube socket may be as little as about 1 mm, with the corresponding outer diameter of the electrode assembly being about 0.1 mm smaller in its outer diameter. Accordingly, a radially expanded weld nugget can prevent the electrode assembly from being inserted axially through the tube socket during the manufacturing process, thus rendering it defective.
Accordingly, it is desired to provide an improved process for welding together wire parts without substantially increasing the diameter thereof at the weld joint.